1. Field of the Invention:
This invention relates to the use of an optical fiber, such as a fiber Bragg grating, to sense a physical condition such as strain, temperature gradient, or pressure (sometimes called a measurand) to which the optical fiber is subjected.
2. Description of the Related Art:
This invention uses two well-known fiber gratings, one of which is called a sensor fiber grating herein, and the other of which is called a reference fiber grating herein, see fiber gratings 10 and 21 of FIG. 1.
A fiber grating of this type comprises a generally mid portion of an optical fiber that has a center and concentrically disposed light transmitting core, an intermediate cladding layer, and an outer buffer/jacket layer that operates to protect the optical fiber from physical damage. The fiber grating's core includes refractive index variations that are distributed along its length. These index changes, or variations, cause a portion of an input light beam to be reflected back out of the light entry end of the fiber grating's core. As is well known, at a critical wavelength, or frequency of the input light beam, all of the components of the reflected light, one component for each index change, interfere in a constructive manner, and a strong or high intensity reflected light beam is provided at this critical wavelength. This critical wavelength is called the Bragg wavelength.
As is known, this periodic index variation can be produced along the core of a single-mode optical fiber by illuminating the core with a periodically patterned Ultra Violet (UV) laser beam. Such an optical fiber, having periodic index variations along the length of its core, is known as an optical-fiber-grating, a fiber Bragg grating, or more simply a fiber-grating. Hereinafter, such an optical fiber will be called a fiber grating, or an optical fiber grating.
If one were to plot the amplitude of the refractive index within the fiber grating's core as a function of a distance that is measured along the core's length, the resultant curve would have a periodic variation, the core would have a nominal or average index of refraction, and the core's index of refraction would vary from this average value in a generally sinusoidal manner.
Mathematically, the Bragg wavelength "B" of a fiber grating is described as, B=2Pn.sub.o where P is the grating period P, and n.sub.o is the nominal or average refractive index of the core.
As shown by the above equation, the Bragg wavelength B of a fiber grating is shifted by a change in the grating period P, and/or by a change in the average refractive index no of the core. In general, the grating period and the average index of refraction change when the fiber grating is subjected to a change in strain (i.e., the fiber grating is stretched lengthwise), a change in length that is caused by a temperature change, or a change in length that is caused by pressure, all of which can be collectively defined as photoelastic and thermo-optic effects.
Both the peak reflectivity and the spectral bandwidth of a Bragg wavelength reflected light beam are functions of the fiber grating's length and the amount of refractive index variation that is present at each periodic index variation that exists along the fiber grating's core. While the two characteristics peak reflectivity and spectral bandwidth are related to the sensitivity of the construction and arrangement of the present invention, they have no effect on the basic function of the present invention.
As a result of the above properties of a well-known fiber grating, a fiber grating can be used as a strain sensor, and/or a temperature sensor, and/or a pressure sensor. In each case, the measurand strain/temperature/pressure is measured by measuring, or determining a shift in the fiber grating's Bragg wavelength from a calibration point, or by measuring the absolute value of the Bragg wavelength. In either event, knowledge of Bragg wavelength enables measurement of the measurand strain/temperature/pressure.
One known way to measure the Bragg wavelength of a fiber-grating is to use an optical spectrum analyzer that draws the wavelength spectrum of the fiber grating's reflected light beam or transmitted light beam. Using this instrument, the fiber grating's Bragg wavelength can be directly read as the center wavelength of the drawn wavelength spectrum. However, this use of an optical spectrum analyzer is expensive, slow, and requires the use of bulky equipment.
An interferometer, such as a fiber Fabry-Perot or a fiber Mach-Zehnder, can also be used to measure the shift of the fiber grating's Bragg wavelength, but this device is also expensive, and it is difficult to calibrate.
Conventionally, strain is also measured by means of an electrical resistance change, or perhaps by measuring an electrical capacitance change, of a device that is generally printed on a thin resin film. Generally, these sensors are designed for surface mounting only, and these sensors have a poor dynamic range due to the nonelastic nature of the films.
U.S. Pat. Nos. 4,915,468 and 5,208,877 are of general interest. Patent '468 describes photo induced reflective changes that are produced in a two-mode, elliptical-core, vibration-modal optical fiber strain sensor. Patent -877 describes a two-mode optical waveguide having a non-circular core, one embodiment of which is a strain gauge.
While the above-described apparatus/methods have been generally useful for their limited intended purposes, the need remains in the art of a strain sensing apparatus/method in which the magnitude of a strain-causing physical condition at the location of a sensor grating is detected by processing a beam output of the sensor grating by the use of a reference grating whose Bragg wavelength is controlled so as to be matched to the Bragg wavelength of the sensor grating.